Fluid flow control valves come in a variety of designs. All have an inlet port receiving pressurized fluid whose flow to an outlet port is to be controlled in some way. The outlet port is to be connected to some device that uses the fluid, and for which the rate of fluid flow must be controlled. Where the fluid is a fuel, the outlet port is usually connected to a burner of some kind.
A valve seat is interposed between the inlet and outlet ports of such a valve. A main valve element moves against the seat to close off fluid flow, and away from the seat to allow fluid to flow from the inlet to the outlet port. Such a valve need not operate to shut off fluid flow completely when closed. Such modulating valves can in one way or another, provide for a range of flow levels as the valve element spacing from the seat is changed. Manually controlled gas valves found on nearly every gas stove are a common type of such a valve. These valves allow flow to be adjusted from completely off, to the minimum needed to maintain a flame, to full flow for high heat output.
Certain types of valves are not operated manually. Some of these, called servo-valves, use pressure of the inlet fluid to provide some of the force required to position the main valve element. One type of such a prior art valve is shown in FIG. 1.
The prior art valve 10 of FIG. 1 is shown in cross section with a body 12 as indicated at a number of places. An inlet port 14 receives high-pressure fluid, which can flow into an inlet chamber 18 in flow communication with port 14.
Fluid can flow through the space between a main valve element 23 and a main valve seat 22 to an outlet chamber 19. From outlet chamber 19, fluid flows through an outlet port 15 to a user device such as a burner. Valve 10 is shown in its open position with main valve element 23 spaced from main valve seat 22. The user device has a known pressure drop from outlet chamber 19 to atmospheric.
Main valve element 23 is carried on a relatively rigid central section of a main valve diaphragm 26. Diaphragm 26 forms a part of the surfaces defining a main diaphragm chamber 30. Main diaphragm 26 forms a fluid-tight seal preventing flow of any fluid directly from inlet chamber 18 to main diaphragm chamber 30. Diaphragm 26 has a flexible periphery with a fluid-tight attachment to the interior surface defining chambers 18 and 30.
A main diaphragm spring 28 within main diaphragm chamber 30 applies force against the center of main diaphragm 26 urging main valve element 23 toward seat 22. Spring 28 has a spring rate constant that causes spring force applied to diaphragm 26 to increase as spring 28 is more fully compressed.
It is convenient to refer to the side of main diaphragm 26 (or any pressure-operated diaphragm) carrying valve element 23 or other valve element as the valve side. The side opposite the valve side of main diaphragm 26 is the control side. It is also convenient to refer to the state or position of a valve element as “open” when shifted as far away from its seat as the system allows, and closed when sealing its seat. A valve element is “partly open” or “partially open” if in a position between open and closed.
Three forces control the main valve element 23 position. This is typically true for any valve element carried on a diaphragm, although some diaphragm-operated valve elements may lack one of these forces. The first of the three forces is the force of the fluid pressure on the valve side of the diaphragm. The second force is the fluid pressure on the control side of the diaphragm. For main diaphragm 26, these pressures are respectively that of the pressure in inlet chamber 18 and on main valve element 23, and the pressure in the main diaphragm chamber 30. The third force is provided by a diaphragm spring such as main diaphragm spring 28.
A servo valve comprising a servo valve element 51 and a servo valve seat 52 controls position of main valve element 23. An electrically operated valve actuator 46 carries servo valve element 51 and can shift element 51 between the open position shown and a closed position with servo valve element 51 pressed against servo valve seat 52. When servo valve element 51 is closed, main valve element 23 is closed as well. Typically, valve actuator 46 includes an internal spring that biases servo valve element 51 so that when actuator 46 does not receive electrical power the spring forces servo valve element 51 to the closed position.
Servo valve element 51 controls fluid flowing through duct 35 to a regulator valve 62 through a duct 54. A regulator diaphragm 55 and spring 61 cooperate to control the position of regulator valve 62 as the pressure in regulator chamber 53 varies. Diaphragm 55 and valve 62 control pressure in main valve diaphragm chamber 30, thereby controlling outlet chamber 19 pressure. The pressure in regulator chamber 53 is held to the pressure in the outlet chamber 19 by the flow communication between chambers 53 and 19 through regulator duct 57.
The regulator diaphragm 55 prevents fluid flow from chamber 53 into the space occupied by the regulator spring 61 and a pressure adjustment screw 60. Adjustment screw 60 can change the spring force applied to regulator assembly 55. The pressure at outlet port 15 increases when screw 60 is turned to shorten spring 61 and thereby increase the force applied urging regulator valve 62 to open further.
Screw 60 forms an airtight seal with body 12 that could interfere with the operation of regulator diaphragm 55. A flow-restricting duct 59 in body 12 bleeds air between the atmosphere and the spring side chamber 65 of regulator diaphragm 55, thereby maintaining atmospheric pressure in chamber 65 while at the same time slowing somewhat (damping) the response of regulator diaphragm 55 to pressure changes in chamber 53.
For a servo valve 10 to operate, an appreciable pressure drop across valve element 23 is required. This pressure drop across main valve element 23 may be approximately 10-40% of the gauge pressure (absolute pressure less atmospheric pressure) at inlet chamber 18, and is adjustable in the embodiment shown. The sum of the pressure drops across main valve element 23 and the user device equals the gauge pressure at inlet chamber 18.
In explaining the operation of FIG. 1 and the other FIGS., the flow of fluid from inlet port 14 to outlet port 15 is indicated by relatively heavy arrows at 16 and 17. Fluid flows for controlling or affecting the position of main valve element 23 (other than the fluid pressure in inlet chamber 18) are shown with thinner arrows, as at 16a and 34.
Closing servo valve seat 52 with servo valve element 51 causes main valve element 23 to close main valve seat 22. When valve seat 52 is closed, fluid pressure in inlet chamber 18 communicates through flow restrictor 33 and duct 35 with servo chamber 43 as shown by arrow 16a. In this way, servo valve element 51 acts to allow pressure in main valve chamber 30 to equalize with pressure in inlet chamber 18.
The result is that the fluid force applied to each side of main diaphragm 28 becomes approximately equal. (In fact, because the net pressure sensed by valve element 23 is that of inlet chamber 18 less the smaller outlet chamber 19 pressure, a small amount of fluid-generated pressure urges valve element 23 toward seat 22.) Force of spring 28 then closes main valve element 23. Spring 28 by itself cannot generate enough force to close valve element 23 against the fluid pressure in inlet chamber 18, but with essentially equal pressure on each side of main diaphragm 26, spring 28 is sufficient to close element 23.
Similarly, whenever servo valve element 51 closes servo valve seat 52, the pressure on each side of regulator diaphragm 55 is equal because pressure equalizes through valve 62 and is essentially atmospheric on each side of regulator assembly 55 as pressure in outlet chamber 19 equalizes with atmospheric through the user device. The regulator spring 61 then holds regulator valve 62 fully open.
When main valve element 23 is to open, valve actuator 46 lifts servo valve element 51 away from servo seat 52. Fluid flows as shown by arrow 44 from chamber 43 to chamber 53 through, servo valve seat 52, duct 54, and open regulator valve 62 to the atmospheric pressure in outlet chamber 19, causing the pressure in chamber 43 to fall well below that in inlet chamber 18. The reduced chamber 43 pressure is communicated through duct 41 to main diaphragm chamber 30, causing the pressure in main valve chamber 30 to equalize with that in servo chamber 43. The reduced chamber 30 pressure causes the net pressure force on main diaphragm 26 to exceed spring 28 force, causing valve element 23 to open.
Pressurized fluid in inlet chamber 18 then begins to flow through valve seat 22 into outlet chamber 19 as shown by arrows 16 and 17. The fluid flowing into outlet chamber 19 from the higher-pressure inlet chamber 18 increases the pressure in outlet chamber 19 from the initial near-atmospheric level.
The pressure drop from inlet chamber 18 to outlet chamber 19 across main valve element 23 along with the pressure drop through the user device holds the pressure in outlet chamber 19 substantially higher than atmospheric. A rule of thumb that often produces satisfactory pressure drop across valve element 23 to operate in full open mode is a total flow area defined by the spacing between element 23 and valve seat 22 that is equal to or less than the area of the opening defined by valve seat 22.
The reduced pressure in servo chamber 43 allows fluid to flow from inlet chamber 18 through flow restrictor 33 into servo chamber 43 as shown by arrows 16a and 34. The fluid flow rate through flow restrictor 33 equals the fluid flow through regulator valve 62 The flow rate through flow restrictor 33 is strictly a function of the pressure difference across flow restrictor 33, and increases with increased pressure drop across flow restrictor 33.
Regulator valve 62 has two purposes. One is to allow the pressure in outlet chamber 19 to be set to a preselected value by adjusting screw 60. The second is to maintain approximately constant pressure in outlet chamber 19 regardless of fluctuations in inlet chamber 18 pressure or user device pressure drop. No regulation of outlet chamber 19 pressure occurs in response to inlet chamber 18 pressure variations without an active regulator valve 62.
The fluid flow rate through regulator valve 62 is a function of the pressure difference across regulator valve 62 as well as the position or setting of valve 62. Main diaphragm chamber 30 pressure equals servo chamber 43 pressure. The setting of regulator valve 62 is a function of the regulator chamber 53 pressure and the regulator spring 61 force. For every setting of valve 62 the resulting pressure in servo chamber 43 and main diaphragm chamber 30 must be a value that results in equal flow through flow restrictor 33 and servo valve 62. The pressure in servo chamber 43 and main diaphragm chamber 30 rises and falls to maintain this equal flow condition at all times.
If outlet chamber 19 pressure decreases slightly for some reason, all other conditions remaining unchanged, then flow through flow restrictor 33 and servo valve 62 increases. The pressure drop across flow restrictor 33 then increases and pressure in servo chamber 43 and main diaphragm chamber 30 falls as well. In addition, decreased outlet chamber 19 pressure causes regulator diaphragm 55 to open regulator valve 62 slightly, decreasing pressure drop across valve 62.
Main diaphragm 26 responds to this lower main diaphragm chamber 30 pressure and shifts main valve element 23 further from seat 22. Pressure drop across main valve element 23 then falls, causing pressure in outlet chamber 19 to rise, restoring the fall in outlet chamber 19 pressure since the net pressure force on regulator diaphragm 55 is referenced to atmospheric.
If inlet chamber 18 pressure should for example fall, regulator valve 62 acts to maintain the selected outlet chamber 19 pressure. Without regulator valve 62, the net pressure difference across main diaphragm 26 changes in a way that is difficult or impossible to predict. On the one hand, the lower inlet chamber 18 pressure acts to allow main valve element 23 to close further, exacerbating the effects of the reduced inlet chamber 18 pressure.
On the other hand, the reduced inlet chamber 18 pressure causes the force in main diaphragm chamber 30 on main diaphragm 26 to decrease as well, causing main valve element 23 to move to a new position that may not compensate for the reduced inlet chamber 18 pressure. The net pressure change on main diaphragm 26 is likely to be uncertain. Accordingly, little or no correction of the drop in outlet chamber 19 pressure occurs without a functioning regulator valve 62.
But with an active regulator valve 62, any pressure drop in outlet chamber 19 regardless of the cause, causes regulator valve 62 to open slightly and the pressure in servo chamber 43 to fall. The lower pressure in servo chamber 43 and main diaphragm chamber 30 causes main valve element 23 to open further from seat 22. The pressure drop across seat 22 falls, raising the outlet chamber 19 pressure and, compensating for the fall in inlet chamber 18 pressure. An increase in inlet chamber 18 pressure induces main valve element 23 to close slightly and reduce outlet chamber 19 pressure. Regulator valve 62 thus compensates for any change in the outlet chamber 19 pressure, to restore that pressure to the preset level.
As a second example of how outlet chamber 19 pressure is sustained at the selected level, consider if at some point, outlet chamber 19 pressure increases for some reason. The increased outlet chamber 19 pressure is communicated to regulator chamber 53 through duct 57 closing regulator valve 62 somewhat and increasing the pressure drop across valve 62. The increased pressure drop across regulator valve 62 increases pressure in servo chamber 43 and main valve chamber 30, causing main valve element 23 to close slightly. When main valve element 23 closes slightly, the pressure drop across main valve seat 22 increases, reducing the pressure in outlet chamber 19 to compensate for the increased outlet chamber 19 pressure.
Outlet chamber 19 pressure can be set to any of a range of values by turning screw 60, and increasing or decreasing the compression of spring 61. The position of regulator valve 62 is controlled by the pressure in regulator chamber 53, which equals the pressure in outlet chamber 19, and by the force of spring 61 opposing the pressure force on diaphragm 55. Additional compression of spring 61 by turning screw 60 further into body 12 results in higher outlet chamber 19 pressure.
To understand this, consider a situation where regulator valve 62 is positioned at a point yielding a particular outlet chamber 19 pressure. If screw 60 is turned to compress spring 61 an additional amount, regulator valve 62 will open further. With valve 62 more open, the pressure drop across valve 62 is smaller. This reduces the pressure in servo chamber 43 and main valve chamber 30. The reduced pressure in main valve chamber 30 results in main valve element 23 opening further and increasing outlet chamber 19 pressure.
Flow restrictor 59 controls flow of air to and from chamber 58 as the position of regulator diaphragm 55 changes. Flow restrictor 59 is selected with a size that provides damping of changes in regulator diaphragm 55 and avoids instability.
One sees from this explanation that an actuator 46 can with relatively small force use the pressure at inlet port 14 to control opening and closing of main valve element 23. At the same time, regulator valve 62 and regulator diaphragm 55 uses pressure at inlet port 14 to hold the outlet chamber 19 pressure relatively constant over a range of inlet chamber 18 pressure.